Scattering coat

ABSTRACT

Diffusing layer based on mineral particles, intended to make a light source homogeneous, characterized in that it incorporates an electromagnetic insulating device whose resistance per square is greater than 100 Ω.

The invention relates to improvements made to a diffusing layer intendedto be deposited on a substrate in order to make a light sourcehomogeneous.

Although the invention is not limited to such applications, it will bedescribed more particularly with reference to layers used to make thelight emitted from a backlighting system homogeneous.

Such a system may in particular be a light source or backlight usedespecially as a backlighting source for liquid crystal screens. Theinvention may also be used when the light from architectural flat lampsused for example in ceilings, floors or walls needs to be madehomogeneous. It can also be used in flat lamps for municipalapplications such as lamps for advertising panels or lamps able toconstitute the shelves or backs of display cabinets.

The light sources used in these backlighting systems are mainlydischarge tubes or bulbs commonly known as CCFLs (Cold CathodeFluorescent Lamps), HCFLs (Hot Cathode Fluorescent Lamps) or DBDFLs(Dielectric Barrier Discharge Fluorescent Lamps). All these systems havein common the fact that they are powered by a variable-voltage sourcethe frequency of which is generally in the range from 10 to 100 kHz.

Now, in these frequency ranges, both in the transient switchon andswitchoff phases and in the steady state phases, electromagneticdisturbances and/or phenomena of the accumulation of surface chargesarise, generating disturbances in the liquid crystal cells.

In order to limit or even eliminate these phenomena it is known practicefor insulation to be provided against the electromagnetic waves createdby the backlighting system and for the surface charges to be removed tothe ground potential of the screen module.

It will be recalled that a screen of this type incorporates, between thebacklighting system (which constitutes the generator of electromagneticinterference) and the LCD (liquid crystal display) screen, a diffusinglayer which, as its name suggests, ensures homogeneous diffusion of thelight source coming from the backlighting systems.

In order to electromagnetically insulate such a screen, use is made, onthis diffuser (which generally is made of plastic, for example of PMMAor of polycarbonate), of a sheet of thermoplastic (PET) which is itselfcovered with a layer of a conducting material, of the ITO (indium tinoxide) type, for example.

Other electromagnetic insulation techniques are known, but these areinappropriate to this type of application. In particular, the use of anarray of conducting wires, or of a metal grating, a metal film, isimpossible. This is because the diffusers incorporating this type ofinsulating device are unable to guarantee a light transmission T_(L) ofat least 50% and a light absorption A_(L) of less than 15%, these twoconditions being required by manufacturers of screens incorporatingbacklighting systems as described above.

In addition, the nature of the material of which the diffuser is madecan be quoted by way of a drawback. We have seen that this diffuser wasgenerally made of plastic. Now, such materials are sensitive to heatand, for large sized screens, measuring more than 10″ across thediagonal (the diagonal in this case being a characteristic dimension ofthe screen), the light sources are situated inside an enclosure as closeas possible to the diffusing part (structure of the direct light type),and this is not generally the case of small-sized screens (measuringless than 10″ across the diameter) for which the light sources arepositioned on the side of the enclosure (structure of the edge lighttype), the light being conveyed toward the diffusing layer by awaveguide, the release of heat being particularly appreciable.

For these large-sized screens, this release of heat generally leads tostructural deformation of the diffusing part, which is embodied byheterogeneity of the brightness of the picture projected onto thescreen.

Aside from these problems of mechanical integrity of the diffusing part,there is also the problem of the additional thickness of the latter dueto the presence of the thermoplastic sheet provided with itselectromagnetic insulating device which leads, on the one hand, tomultiple reflections and, on the other hand, to additional cost at thetime of assembly.

Now, the current desire which is tending toward reducing the size ofscreens in terms of their thickness and in terms of the number ofcomponents involved goes against this solution. Furthermore, thisincrease in thickness leads to a reduction in brightness of theprojected picture.

The inventors have therefore set themselves the task of finding a meansof obtaining an electromagnetic insulation for a large-sized screen(measuring more than 10″ across the diagonal) and which does not havethe disadvantages of the aforementioned solutions, particularly in termsof the size and in terms of the loss of picture quality.

To this end, the diffusing layer based on mineral particles, intended tomake a light source homogenous, is characterized, according to theinvention, in that it incorporates an electromagnetic insulating devicewhose resistance per square is greater than 100 Ω.

In some preferred embodiments of the invention, recourse may possiblyalso be had to one and/or other of the following arrangements:

-   -   the resistance per square is between 300 and 700 Ω,    -   the insulating device consists of at least one layer that is        translucent in the visible domain and made of electrically        conducting material, said conducting layer being deposited as        close as possible to the diffusing layer,    -   the conducting layer is based on translucent conducting oxide,    -   the diffusing layer is deposited on a substrate and the        conducting layer is deposited on said diffusing layer,    -   the diffusing layer is combined with a substrate, the conducting        layer being placed between the substrate and the diffusing        layer,    -   the diffusing layer is combined with a substrate, the diffusing        layer being deposited on one of the sides of the substrate,        while the conducting layer is deposited on the opposite side of        said substrate,    -   the insulating device is incorporated into the diffusing layer,    -   the diffusing layer is made of elements comprising particles and        a binder, the binder allowing the particles to be agglomerated        with one another, the insulating device consisting of one or        other of said elements,    -   the particles are made of metal or metal oxides,    -   it contains particles of ZrO₂,    -   the particle size is between 50 nm and 1 μm,    -   the particles are based on F:SnO₂ or ITO    -   the binder is a mineral or organic electrically conducting        binder,    -   the substrate is a glass substrate,    -   the substrate is a transparent substrate based on polymer, for        example made of polycarbonate,    -   the diffusing layer incorporates a coating having a        functionality other than that of insulating, particularly a        coating with a low-emissivity function, antistatic function,        antifogging function or an antifouling function.

According to another aspect of the invention, this invention targets theuse of a diffusing layer as described hereinabove to produce a diffusingsubstrate in a backlighting system and/or flat lamp system.

In some preferred embodiments of the invention, recourse may possiblyalso be had to one and/or other of the following arrangements:

-   -   the substrate is one of the sheets of glass that make up the        backlighting system and/or of a flat lamp,    -   the substrate has a characteristic dimension tailored to direct        light applications,    -   the thickness and/or the cover density of the layer varies over        the deposition surface,    -   the thickness of the diffusing layer is between 0.5 and 5 μm.

Other advantages and particulars of the invention will become apparentfrom reading the detailed description which will follow.

Thus, according to a first embodiment of the invention, the diffusinglayer consists of particles agglomerated in a binder, said particleshaving a mean diameter of between 0.3 and 2 microns, said binder beingin a proportion of between 10 and 40% by volume and the particlesforming aggregates the dimension of which ranges between 0.5 and 5microns, said layer having a contrast attenuation greater than 40% andpreferably greater than 50%. This diffusing layer is particularlydescribed in application WO 0190787.

The particles are chosen from semitransparent particles and preferablyfrom mineral particles such as oxides, nitrides and carbides.

The particles will preferably be chosen from the oxides of silica,alumina, zirconia, titanium, cerium or a mixture of at least two ofthese oxides.

Such particles may be obtained by any means known to those skilled inthe art particularly by precipitation or by pyrogenation. The particleshave a particle size such that at least 50% of the particles deviatefrom the mean diameter by less than 50%.

The binder has sufficient temperature withstand to withstand theoperating temperatures and/or the sealing temperature of the lamp if thelayer is produced before the lamp is assembled and in particular beforethe latter is sealed.

When the layer is in an exterior position, the binder is also chosen tohave enough resistance to abrasion that it can, without damage, undergoall the handling of the backlighting system, for example when mountingthe flat screen.

Depending on the requirements, the binder may be chosen to be mineral,for example in order to encourage temperature resistance in the layer,or organic, particularly to simplify the production of said layer, itbeing possible for crosslinking to be obtained simply, for example inthe cold state. The choice of a mineral binder whose temperatureresistance is high will in particular make it possible to produce abacklighting system with a long life without any risk of any degradationof the layer occurring due, for example, to fluorescent tubes whichproduce considerable heating. Indeed it has been found that, with theknown solutions, there is degradation of the plastic film withtemperature and this therefore makes producing large-size backlightingsystems an enormously tricky prospect.

The binder has an index different than that of the particles and thedifference between these two indexes is preferably at least 0.1. Theindex of the particles is above 1.7 and that of the binder is preferablybelow 1.6.

The binder is chosen from the calcium silicates, sodium silicates,lithium silicates, aluminum phosphates, polymers of the polyvinylalcohol type, thermosetting resins, acrylics, etc.

To encourage the formation of aggregates in the desired size, theinvention anticipates the addition of at least one additive leading to arandom distribution of the particles in the binder. As a preference, theadditive or dispersant is chosen from the following: an acid, a base, orionic polymers of low molecular mass, particularly of a mass less than50 000 g/mol.

It is also possible to add other agents, for example a wetting agentsuch as nonionic, anionic or cationic surfactants, to form a layer whichis homogeneous on a large scale.

It is also possible to add rheology modifiers such as cellulose ethers.

The layer thus defined may be deposited with a thickness of between 1and 20 microns. The methods for depositing such a layer may be any meansknown to those skilled in the art such as depositing by screenprinting,coating with paint, dipcoating, spincoating, flowcoating, spraying, etc.

When the desired thickness of the deposited layer is greater than 2microns, a deposition process of the screen-printing type is used.

When the thickness of the layer is less than 4 microns, deposition ispreferably performed by flowcoating or by spraying.

Provision is also made for the production of a layer whose thicknessvaries according to the area of coverage on the surface; such anembodiment may allow intrinsic inhomogeneities in a light source to becorrected. For example, it is possible in this way to correct thevariation in illumination of light sources along their length. Accordingto another embodiment leading to practically the same effect ofcorrecting for intrinsic inhomogeneities of light sources, provision ismade for there to be a layer whose cover density varies over thedeposition surface; this may, for example, be a coating deposited byscreenprinting the density of spots of which varies from a completelycovered region to a region of dispersed spots, the transition beinggradual or otherwise.

According to another embodiment of the diffusing layer, provision ismade for at least one of the elements, or even at least two of theelements that make up the diffusing layer to be electrically conducting.These may either be particles forming the aggregates or particlesforming the binder.

In the case of an electrically conducting binder of SnO₂ mineral ororganic type, provision is for example made for use to be made of aconducting polymer (polypyrrole) or nanoparticles (F:SnO₂, Sb:SnO₂,ITO).

When the particles that form the aggregates are electrically conducting,these may be based on transparent conducting oxide powder such asF:SnO₂, Sb:SnO₂, Sn:In₂O₃, Al:ZnO, for example.

According to yet another embodiment, the diffusing layer may be obtainedfrom a substrate which has undergone a surface treatment. This may forexample be a sand-blasted substrate, a substrate which has undergone anacid attack marketed by Saint Gobain Glass France under the name of“Satinovo”®, or alternatively a substrate coated with a coat of enamelmarketed by Saint Gobain Glass France under the names “Emalit”® or“Opalit”®.

Regardless of the embodiment of the diffusing layer (except for the oneobtained from intrinsically electrically conducting elements), thislayer needs to be combined with a device that provides electromagneticinsulation and/or provides for the flow of surface charges.

This electromagnetic insulating device consists of at least oneelectrically conducting layer positioned as close as possible to thediffusing layer, this conducting layer being transparent in the visibledomain (including having low or zero haze, in this case beingtranslucent).

According to the invention, such conducting layers are deposited ontransparent or semitransparent substrates having a flat or non-flatshape depending on the applications.

The conducting layer is made up of conducting transparent oxides (morecommonly known as TCOs) such as, in particular, F:SnO₂, Sb:SnO₂,Sn:In₂O₃, Al:ZnO.

According to a first technique, this conducting layer can be producedusing a reactive cathode sputtering process either from metal targets orfrom oxide targets.

According to a second technique, the conducting layer may be producedusing a pyrolytic technique.

This may involve the pyrolysis of powder. This technique consists inusing a jet of carrier gas to spray onto the surface of the substrate, apowder of organometallic precursors or a mixture of powders, the powderbreaking down under the effect of the heat of the substrate, releasingthe atoms that make up the conducting layer.

It may also involve the pyrolysis of liquid. According to this process,the chemical precursors, in the form of a liquid solution or suspension,are brought into contact with the substrate for example using aspraycoating technique or a dipcoating or spincoating technique.

The conducting layer may also be deposited on the substrate by chemicalvapor deposition (CVD) or by plasma-enhanced CVD.

According to yet another technique, the conducting layer may be obtainedby a sol-gel technique.

Whatever the method used to produce the conducting layer, the latter hasa resistance per square of more than 100 Ω and preferably of between 300and 700 Ω. This conducting layer constitutes an insulating device forfrequencies of between 10 and 100 kHz; this conducting layer also makesit possible to produce a device for the flow of electrostatic or surfacecharges. (These resistance per square properties are also obtained bythe intrinsically conducting diffusing layer described hereinabove).

This conducting layer is therefore associated with a diffusing layer,the whole being associated with a substrate, particularly one made ofglass or of polymer (PMMA, polycarbonate).

This association with the substrate may be achieved in various ways:

-   -   the substrate is placed between the diffusing layer and the        conducting layer,    -   the conducting layer covers one of the sides of the substrate,        the diffusing layer for its part covering the conducting layer,    -   the diffusing layer covers one of the sides of the substrate,        the conducting layer for its part covering the diffusing layer,    -   the diffusing layer comprising at least one electrically        conducting element (binder and/or aggregate) is in contact with        one of the sides of the substrate.

Whatever the configuration of the association formed by the substrate,the diffusing layer alone (intrinsically conducting), the diffusinglayer associated with the conducting layer, the assembly has a lighttransmission T_(L) of at least 20%, and preferably of more than 50% anda light absorption A_(L) of less than 15%. The thickness of thediffusing layer thus formed is between 0.5 and 5 μm, of which 10 nm to 1μm account for the single conducting layer. The light transmission valuefor the conducting layer alone is at least 80% and preferably above 85%.

An alternative form of embodiment which can be associated with themethods of producing diffusing layers having a shielding devicedescribed hereinabove, consists in incorporating into the assembly acoating which has another functionality. This may be a coating with afunction of blocking out radiation with wavelengths in the infrared(using, for example, one or more layers of silver surrounded by layersof dielectric, or layers of nitride such as TiN or ZrN or of metaloxides or of steel or of Ni—Cr alloy) with a low emissivity function(for example using a doped metal oxide such as F:SnO₂ or tin-dopedindium oxide ITO or one or more layers of silver), a heating layer(doped metal oxide, for example Cu, Ag) or an array of heating wires(copper wires or strips screen-printed from a conducting silver slurry),an antifogging function (using a hydrophilic layer) an antifoulingfunction (photocatalytic coating containing TiO₂ at least partiallycrystallized in anotase form).

The applications for which the invention is intended are, in particular,backlighting systems for example used for backlighting liquid crystaldisplay screens, or alternatively flat lamps used for architecturallighting or alternatively municipal lighting, or more generally in anysystem incorporating light sources likely to generate electromagneticdisturbances.

In the nonlimiting case of flat lamps, the assembly of layers (diffusingplus electrically conducting layers) is deposited on the sheet of glassthat constitutes the front face of the lamp.

According to a first embodiment of a flat lamp that is to incorporatethe diffusing layer according to the invention, the collection of layers(diffusing plus electrically conducting layers) is deposited on the sideof the sheet of glass that faces toward the inside of the lamp; in suchan embodiment, the collection of layers (diffusing plus electricallyconducting layers) is to be deposited on a sheet of glass while the lampis being produced. According to this embodiment, the collection oflayers has to have enough temperature resistance to withstand thevarious heat treatments needed to produce such a lamp, particularly tocarry out the deposition activities that correspond to the production ofthe electrodes and to seal around the periphery of the two sheets ofglass that make up the structure of the flat lamp.

If spacers are needed, particularly to keep a uniform space between thetwo sheets of glass, the invention provides for the collection of layers(diffusing plus electrically conducting layers) to be deposited leavingfree regions corresponding to the locations intended for the spacers sothat the adhesion of these spacers is not disturbed by the layeraccording to the invention. Such free spaces may easily be obtained bychoosing to deposit the layer using a screen-printing technique.

According to a second embodiment of a flat lamp incorporating thediffusing layer according to the invention, the layer (diffusing pluselectrically conducting) is deposited on the side of the sheet of glassfacing toward the outside of the lamp; according to this embodiment thecollection of layers (diffusing plus electrically conducting layers) ischosen to have enhanced mechanical resistance properties, particularlyenhanced resistance to abrasion.

According to yet another alternative form of embodiment regarding theuse of the collection of improved diffusing layers according to theinvention (diffusing plus electrically conducting layers) in theembodiment of a flat lamp and/or of a backlighting system, said layer(diffusing and electrically conducting) is deposited on a transparent orsemitransparent substrate independent of the sheets of glass that makeup the structure of the flat lamp or of the backlighting system. Such anembodiment may consist in depositing the collection of layers (diffusingplus electrically conducting layers) on a glass substrate held somedistance away from the front face of the lamp or of the backlightingsystem; this embodiment makes it possible, according to the rules ofphysics, to further improve the diffusing effect of the collection oflayers. In counterbalance, the volume or bulk of such an embodiment onceagain becomes equivalent to the solutions known in the prior art, butwith diffusion and electromagnetic insulation performance that is farmore durable over time.

Improved layers (diffusing and insulated) thus set out in accordancewith the invention therefore make it possible to produce backlightingsystems for example intended for illuminating liquid crystal displayscreens. By comparison with the solutions known in the prior art, thelayer according to the invention makes it possible to reduce the bulk ofsaid backlighting system for a given performance in terms of luminance,brightness and life.

1. A diffusing layer comprising a mineral particle layer and anelectromagnetic insulating device with a resistance per square greaterthan 100 Ω.
 2. The diffusing layer as claimed in claim 1, wherein theelectromagnetic insulating device has a resistance per square between300 and 700 Ω.
 3. The diffusing layer as claimed in claim 1, wherein theelectromagnetic insulating device consists of at least one electricallyconducting layer that is translucent in the visible domain, saidconducting layer being deposited as close as possible to the mineralparticle layer.
 4. The diffusing layer as claimed in claim 3, whereinthe conducting layer comprises a transparent conducting oxide.
 5. Thediffusing layer as claimed in claim 1 wherein the mineral particle layeris deposited on a substrate and the conducting layer is deposited onsaid mineral particle layer.
 6. The diffusing layer as claimed in claim1 wherein the mineral particle layer is combined with a substrate, theconducting layer being placed between the substrate and the mineralparticle layer.
 7. The diffusing layer as claimed in claim 1 wherein themineral particle layer is combined with a substrate, the mineralparticle layer being deposited on one of the sides of said substrate,while the conducting layer is deposited on the opposite side of saidsubstrate.
 8. The diffusing layer as claimed in claim 1 wherein theelectromagnetic insulating device is incorporated into the mineralparticle layer.
 9. The diffusing layer as claimed in claim 1 wherein themineral particle layer further comprises a binder, the binder allowingthe mineral particles to be agglomerated with one another.
 10. Thediffusing layer as claimed in claim 9, wherein the mineral particlecomprises metal or metal oxides.
 11. The diffusing layer as claimed inclaim 9, wherein the mineral particle comprises ZrO₂.
 12. The diffusinglayer as claimed in claim 9 wherein the mineral particle size is between50 nm and 1 μm.
 13. The diffusing layer as claimed in claim 9 whereinthe mineral particle comprises F:SnO₂ or ITO.
 14. The diffusing layer asclaimed in claim 9, wherein the binder is a mineral or an organicelectrically conducting material.
 15. The diffusing layer as claimed inclaim 5 wherein the substrate is a glass substrate.
 16. The diffusinglayer as claimed in claim 5 wherein the substrate is a transparentsubstrate comprising a polymer.
 17. The diffusing layer as claimed inclaim 1 wherein the diffusing layer incorporates a coating having afunctionality other than that of insulating, particularly a coating witha low-emissivity function, antistatic function, antifouling function oran antifouling function.
 18. The diffusing layer as claimed in claim 1wherein it has a light transmission T_(L) greater than 20% andpreferably greater than 50%.
 19. The diffusing layer as claimed in claim1 wherein it has a thickness of between 0.5 and 5 μm.
 20. A method forproducing a diffusing substrate in a system provided with light sourcescomprising adding a diffusing layer as claimed in claim 1 to a diffusingsubstrate in a system provided with light sources.
 21. A method forproducing a diffusing substrate in a backlighting system comprisingadding a diffusion layer as claimed in claim 1 to a diffusing substratein a backlighting system.
 22. The method as claimed in claim 21 whereinthe diffusing substrate is a sheet of glass that comprises thebacklighting system.
 23. A method for producing a diffusing substrate ina flat lamp system comprising adding a diffusion layer as claimed inclaim 1 to a diffusing substrate in a flat lamp system.
 24. The methodas claimed in claim 23 wherein the diffusing substrate is a sheet ofglass that comprises the flat lamp system.
 25. The method as claimed inclaim 20 wherein the diffusing substrate has a characteristic dimensiontailored to direct light applications.
 26. The method as claimed inclaim 20 wherein the thickness and/or the cover density of the diffusionlayer varies over the deposition surface.
 27. The diffusing layer asclaimed in claim 4 wherein the transparent conducting oxide is selectedfrom the group consisting of F:SnO₂, Sb:SnO₂, Sn:In₂O₃, Al:ZnO andmixtures thereof.
 28. The diffusing layer as claimed in claim 6 whereinthe substrate is a glass substrate.
 29. The diffusing layer as claimedin claim 7 wherein the substrate is a glass substrate.
 30. The diffusinglayer as claimed in claim 16 wherein the polymer is polycarbonate. 31.The diffusing layer as claimed in claim 6 wherein the substrate is atransparent substrate comprising a polymer.
 32. The diffusing layer asclaimed in claim 7 wherein the substrate is a transparent substratecomprising a polymer.
 33. A light source comprising the diffusion layeras claimed in claim
 1. 34. A backlighting system comprising thediffusion layer as claimed in claim
 1. 35. A lamp comprising thediffusion layer as claimed in claim 1.